Muscle memory discovery ends 'use it or lose it' dogma. Exercise physiologists agree: muscle memory is real. But how are these 'memories' stored? A review has a simple answer: nuclei gained during training persist even when muscle cells shrink due to disuse or start to break down. This means we can 'bank' nuclei in our youth to prevent frailty in old age -- and makes steroid use in competitive sport a perfect but irredeemable crime. Source 5j.